Down hole drilling units are frequently used for multiple purposes, such as drilling through virgin formation, cleaning a wellbore, drilling through cement plugs, etc. Depending on the task at hand, such downhole drilling units be run on drill strings, wireline cable, or coiled tubing, for example. The cost to drill or service a wellbore may be determined in large part by the effective rate of penetration during drilling operations. Traditional rotating drill bits are useful for shearing and removing weak materials. As well depth increases, formation rock strength may increase, and the mechanical limitations of the drilling string and the drill bits may result in decreased rate of penetration. Similarly, drilling through cement plugs or other downhole tools may result in a low rate of penetration.
Downhole tools that impart axial impact forces to a drill bit may increase rock cutting efficiency while simultaneously reducing the required rock cutting force. Reducing cutting force may result in lower drill bit wear and breakage, less frequently encountered stick-slip conditions, lower probability of shearing the drill string, and a concomitant greater effective rate of penetration. Downhole impact tools that create axial impact forces using a hydraulic flow of drilling fluid that actuate a complex system of valves and pistons may not be particularly optimal for all drilling operations, particularly those operations conventionally performed using wireline or coiled tubing systems.